Sr/Ca sensitivity to aragonite saturation state in cultured subsamples from a single colony of coral: mechanism of biomineralization during ocean acidification

Using a new and rapid NanoSIMS-based method, we quantified the sensitivity of skeletal Sr/Ca in coral to the aragonite saturation state of seawater (ΩSW). Skeletal Sr/Ca is a common proxy for temperature while ΩSW is a parameter that varied in the past ocean and is predicted to change with continued ocean acidification. Five adult branches of the surface coral Stylophora pistillata were grown at different ΩSW from 2.7 to 4.9 (pH of 7.9 to 8.5) but at a constant temperature of 25 °C. Despite a large range of growth parameters and a two-fold range in calcification rates, the average skeletal Sr/Ca of coral exposed to each condition are within 1.2% of each other (2σ std. dev. of the 5 means). Furthermore, the average skeletal Sr/Ca measured in this study agrees with the results of two previous coral culture experiments conducted at the same temperature but where ΩSW was not controlled. These results suggest that aragonite saturation has little or no influence on Sr/Ca paleothermometry over the range of ΩSW investigated. Combined with existing data for low ΩSW conditions, our results were used to elucidate the mechanisms controlling calcifying fluid acid-base chemistry during coral biomineralization. Assuming that coral drive precipitation through alkalinity pumping, our data suggest that this pumping occurs until the calcifying fluid reaches a target pH. Below a threshold ΩSW bounded by 1 < ΩSW < 2.4, however, coral do not pump enough alkalinity to reach the target pH and instead pump a maximal but finite amount of alkalinity. In this low ΩSW regime, calcifying fluid pH is expected to decrease with ΩSW. The interplay between these two alkalinity pumping regimes and external seawater composition explain the full range of observed Sr/Ca sensitivity to ΩSW and suggest that surface coral may become increasingly sensitive to ocean acidification below a threshold ΩSW bounded by 1 < ΩSW < 2.4.